Temperature detector

ABSTRACT

In an optical fiber type temperature detector using temperature change of refraction index in birefringent crystal, by using LiTaO 3  or Sr x  Ba l-x  Nb 2  O 6  single crystal (0.5&lt;x&lt;1.0) as material (13) for sensor and quartz as material for setting temperature range, a high sensitive and highly stable temperature detector using light emitting diode as light source is realized. Furthermore, as configuration of sensor part, miniaturization and light weight is devised by disposing rutile (11) or calcite between optical fibers (8, 9) and rod lens (12) to make polarization separation.

FIELD OF THE TECHNOLOGY

The present invention relates to a temperature detector using opticalfibers, which temperature detector can be utilized in technical field oftemperature measurement of a high tension system apparatuses,long-distance remote temperature measurement, or the like.

BACKGROUND OF TECHNOLOGY

Temperature detector utilizing optical fibers is one of extremely highnecessity as measuring apparatus to replace the conventional electrictemperature detector under bad environment where electromagneticinduction noise or electric insulation comes into problem. Or again, ina remote measuring system utilizing a communication system of low-lossoptical fiber, an element which transfer external physical amount, suchas, temperature directly into light signal without once transforminginto electric signal becomes important one.

FIG. 1 shows a configuration of conventionally proposed temperaturedetector using optical fiber for such use. The temperature detectorconsists of a birefringent crystal 1 for temperature detection, opticalfibers 2, 7, lens 3, 6, sensor part consisting of a polarizer 4 and ananalyzer 5 with their transparency polarization directions having rightangle each other, light emitting part 2 (not shown) connected to theother end of the optical fiber 2, and received light signal processingpart (not shown) connected to the other end of the optical fiber 7.

Operation of the temperature detector of such configuration iselucidated; for instance at a temperature of T₁ the direction ofpolarization of light passing the birefringent crystal 1 does notsubstantially change, accordingly the direction of polarization of lightafter passing the crystal 1 is in the state having right angle to thedirection of transmitted polarized light of the analyzer 5, the lightpassing is hindered by the analyzer 5 and light output intensity of theoptical fiber 7 becomes minimum. Next, as temperature rises thedirection of polarization of light passing the birefringent crystal 1becomes to change during the passing; for instance at the temperature T₂(T₁ <T₂) a part of light which passes the birefringent crystal 1 becomesof the same polarization direction as the transmitted polarized light ofthe analyzer 5 and light output intensity of the optical fiber becomesmaximum. Since the light output of the optical fiber 7 changesresponding to temperature of the birefringent crystal 1 in this manner,by detecting the light intensity the temperature of the part where thebirefringent crystal 1 is located can be known. Incidentally, in thesaid configuration, transparency polarizing direction of the polarizer 4and analyzer 5 are set to have right angle each other, but thesedirections are set parallelly.

In such temperature detector, as the light source a use of lightemitting diode in which characteristic is stable and price is cheap, ispreferable, but in the conventional way of using the light emittingdiode, there have been made no study as to which material for thebirefringent crystal 1 may be used.

Furthermore, there has been made no study as to in which range ofcharacteristic curve of temperature and light output intensity of thebirefringent crystal 1 an operation range should be determined in orderthat a highest accuracy measurement can be made.

Still furthermore, there has been made no study at all as to howmeasurement error due to fluctuation of loss of light passing throughoptical fibers 2, 7 is prevented.

Summarizing the above, although the temperature detector using theoptical fiber is observed hopeful as measurement apparatus usable undera bad condition where electromagnetic induction noise, electricinsulation, etc., are the question, it has not come to actual use sincestudies of the above-mentioned points have not been made so far.

DISCLOSURE OF THE INVENTION

Accordingly, the temperature detector of the present invention, bysufficiently studying the above-mentioned conventional technical points,realized actually very useful temperature detector. That is, temperaturedetector of the present invention uses LiTaO₃ single crystal or Sr_(x)Ba_(1-x) Nb₂ O₆ single crystal (0.5<x<1.0) is used as temperaturedetecting birefringent material with which a light emitting diode ofhigh reliability and stability as light source is usable, is used.

Furthermore, the temperature detector of the present invention usesLiTaO₃ single crystal of Sr_(x) Ba_(1-x) Nb₂ O₆ single crystal(0.5<x<1.0), and temperature range setting element consisting of quartzfor determining optimum operation range of light incident to thesesingle crystals is provided.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is configuration diagram of a conventional temperature detector,

FIG. 2 is a graph showing characteristics of various birefringentcrystals,

FIG. 3 is configuration diagram of a temperature detector in a firstembodiment of the present invention,

FIG. 4 is a graph showing measured results of temperature characteristicof the temperature detector,

FIG. 5 is a configuration diagram of a temperature detector in a secondembodiment of the present invention,

FIG. 6 is a configuration diagram of a temperature detector in a thirdembodiment of the present invention,

FIG. 7 is a configuration diagram of a temperature detector in a fourthembodiment of the present invention.

THE BEST MODE FOR CARRYING OUT THE INVENTION

For a light source for a temperature detector, either a semconductorlaser or a light emitting diode may be used, but the semiconductor laserhas many problems at the present time for a light source for stablemeasurement because of reflection light noise, mode distribution noise,speckle noise, etc. On the contrary, output stability of the lightemitting diode is superior and also it is cheap and suitable as lightsource for temperature detection apparatus. However, since the lightemitting diode has spectral spread of about 30-100 nm, birefringentcrystal which can be utilized for temperature detection becomes limited.As representatives of birefringent crystal for temperature detection isLiNbO₃, LiTaO₃, SiO₂ (quartz) or the like is considered, but in orderthat the light emitting diode having the spectral spread is usable,difference of birefringent indices of the crystal must be small. Thatis, optical retardation φ is represented by the following equation, whenthe birefringent indices are n₁, n₂, and thickness of the crystal is l:##EQU1## Thereupon, with respect to the direction (Z direction) of thecrystal axis of the birefringent crystal, it is provided that the lightis made to pass in the vertical direction.

In the above-mentioned equation in order to decrease change of opticalretardation φ responding to wavelength λ of the light source, it ispreferable that difference of refraction indices (n₁ -n₂) or thickness lof the crystal is preferably small, but, a crystal having smalldifference of refractive indices Δn=n₁ -n₂ becomes necessary since thecrystal thickness l is set from temperature detection sensitivity. Thebelowmentioned table shows refractive indices of various birefringentcrystals, and for crystals of large temperature change rate B and smalldifference of refraction indices LiTaO₃ and SBN are understood to behopeful. There SBN is abbreviation of Sr_(x) Ba_(1-x) Nb₂ O₆, and thestate of x=0.75 is shown in the below-mentioned table.

                  TABLE                                                           ______________________________________                                                                Difference of                                                   Temperature change                                                                          refraction indices                                    Crystal   rate          (Δn)                                            ______________________________________                                        LiNbO.sub.3                                                                             47 × 10.sup.-6 /°C.                                                            0.083                                                 LiTaO.sub.3                                                                             55 × 10.sup.-6 /°C.                                                            0.004                                                 SiO.sub.2  1 × 10.sup.-6 /°C.                                                            0.009                                                 SBN       200 × 10.sup.-6 /°C.                                                           0.013                                                 ______________________________________                                    

Therein, the temperature change rate is given B=d(Δn)/dT.

FIG. 2 shows variation of optical retardation φ responding to wavelengthλ where comparison is made providing that the temperature change rates Bare constant, and it is understood that changes of LiTaO₃ and SBN areextremely smaller compared to LiNbO₃ and SiO₂, and they are usable evenfor light emitting diode having spectral spread. The present inventorfound that single crystal of LiTaO₃ or SBN is extremely splendidmaterial as temperature detection element using a light emitting diode,which is cheap and of good stability, as light source.

FIG. 3 is a configuration block diagram of sensor part of a temperaturedetector of one embodiment of the present invention, wherein to each endface of input and output optical fibers 8, 9, a rutile plate 11 of 1 mmthickness and having optical axis 10 obliquely (here 45° ), rod lens 12,LiTaO₃ single crystal Y-plate 13 of 70 μm thickness and a dielectricmirror 14 are made bonded by means of optical plastics, and the wholeparts are encapsulated in a protection glass tube 15.

Operation of the above-mentioned temperature detector is elucidated:light of the optical fiber 8 connected to the light source of the lightemitting diode is polarization-separated into two lights havingpolarization directions having right angle to each other, and thereafterthey are made into parallel rays by means of the rod lens 12, and whentwice passing turning back through the LiTaO₃ single crystal 13 beforeand after reflection by the dielectric mirror 14, the polarizationdirection receives change responding to temperature in the singlecrystal 13. Then, by means of passing of light through the rod lens 12and rutile plate 11 again, only the light component of the samepolarization as that which is prior to passing the single crystal 13 isoutput at the optical fiber 9, and the light component polarizationdirection of which rotates in the single crystal 13 responding thetemperature, is hindered of passing by the rutile plate 11. Thus, fromthe light amount value of the light output from the optical fiber 9,temperature can be detected. Here the rutile plate is for carrying outpolarization separation, and the same function can be satisfied also byusing calcite, but in view point of mechanical strength rutile issuperior. The angle formed between the cut plane of rutile and opticalaxis needs not necessary be 45°, but within a range of 40°-50° itoperates enough.

Characteristic of the configuration of the temperature detector of FIG.3 is firstly using of LiTaO₃ 13 of stable characteristic as temperaturedetection element, and further decrease of number of used components andminiaturization are intended by making it reflection turn-back type byusing the dielectric mirror 14. Incidentally, rutile plate 11 and rodlens 12 may be only one each. Besides, it is a feature that instead ofusing expensive and large sized Grantomson prism which is hitherto usedto make the polarization separation, such a disposition is adopted thatthe rutile plate 11 with oblique optical axis is between the opticalfibers 8, 9 and rod lens 12, to devise miniaturization and decrease ofcost.

FIG. 4 is experimental results of temperature characteristic oftemperature detector of FIG. 3, and as the light source a light emittingdiode of wavelength λ=0.82 μm is used. As is understood from the samefigure, light output varies sinusoidally responding to the temperature,and is almost linear at around 20° C., and within an operation range Alight output change of about 3.5% per 1° C. is obtainable, and a highaccuracy of less than 0.1° C. as measurement accuracy is contrived.

Incidentally, in FIG. 4 gradation of the characteristic curve, namelytemperature change rate, is proportional to thickness of the crystal,and it is necessary to be designed from necessary measurement accuracyand measurement range. However, since the temperature measurement range,that is positions of maximum and minimum change together withtemperature measurement rate at this time, 20° C. does not necessarilycome on the linear part of the sinusoidal curve as shown in FIG. 4.Accordingly, if an optical bias plate capable of individually settingthe temperature change rate and a reference point (for instance a lightoutput at 20° C.) is inserted, then design of the temperature detectorbecomes very easy.

FIG. 5 shows another embodiment of the temperature detector of thepresent invention wherein, in order to carry out individual designingsof the temperature change rate and the reference point, namelytemperature measurement range, on top of configuration of FIG. 3, aquartz 16 is added as the optical bias plate, and it is an embodimentconfiguration diagram wherein the temperature change rate is set bythickness of the LiTaO₃ 13 and the reference point is set by thicknessof the quartz plate 16. Operation principle of the present embodimentis, utilizing that as is obvious from the preceding table, thetemperature change of quartz is so small as only 2% of the LiTaO₃, andthe temperature change rate is determined almost by LiTaO₃ only, andthickness of the optical bias plate quartz may be determined by thatwhere the reference point is to be set even when complex-configurated.For instance, in order to bring the 2% reference point (20° C.) to thelinear part of the sinusoidal curve, it is enough to make that LiTaO₃ is60 μm, and quartz is 45 μm.

Quartz (Y-plate) only has been taken up as the optical bias plate here,but mica plate or the like birefringent crystal can be used. The opticalbias plate has been used conventionally in electrooptic modulator, butas the temperature detector the present invention is a first case andutility is very great.

FIG. 6 is a construction diagram of a temperature detector in a thirdexample of the present invention, and shows means to prevent abruptchange or hysteresis of the optical output caused by pyro-electriceffect. That is, ferro-electric crystal such as LiTaO₃ or SBN as thetemperature detection material also has the pyro-electric effect, andespecially when the crystal length is great the abrupt change orhysteresis in the optical output is observed. Therefore, as shown inFIG. 6, as a result of vapor-depositing Au-Cr electrodes 17, 18 on andbeneath Z plane of the single crystal 13 of LiTaO₃ or SBN andshort-circuiting between both electrodes 17, 18 it is found that a verystable characteristic is obtainable. Incidentally numerals 19, 19' showdirections of ways.

FIG. 7 is a configuration diagram of a temperature detector in thefourth embodiment of the temperature detector in accordance with thepresent invention, and making of a higher accuracy of the temperaturedetector of FIG. 5 is intended. It is a high accuracy temperaturedetector wherein two different wavelengths λ₁, λ₂ are multipllytransmitted in an input and output optical fibers 8, 9, and a reflectionfilm 20 which reflects only the light of λ₂ between the rod lens 12 andthe quartz plate 16, and it is devised that temperature information isto be contained only in the light of λ₁, and by processing outputintensities of lights of two wavelengths, a decrease of measurementerror due to fluctuation of loss of optical fibers 8, 9 is intended.

In the above, description is made with respect to the temperaturedetectors using LiTaO₃ single crystal, but it is similar for SBN. And,for the composition of SBN, a one of 0.5<x<1.0 is superior incharacteristics in smallness of birefringence and non existence ofhysteresis. Furthermore, the optimum range is 0.7<x<0.8.

INDUSTRIAL UTILITY

The present invention enables actual use of temperature detector usingfor the light source a light emitting diode which is cheap and superiorin output stability, by using LiTaO₃ single crystal or SBN singlecrystal as birefringent crystal. Especially a novel configuration thatSiO₂ (quartz) or the like as optical bias plate is disposed in series tothe light part in order to individually set detection sensitivity(temperature change rate) and reference point is very useful forapplications.

Furthermore, the temperature detector of the present invention isprovided with temperature range setting use one consisting of quartz forhigh accuracy measurement by determining optimum operation range ofincident light to LiTaO₃ single crystal or SBN single crystal.

As has been elucidated, the temperature detector of the presentinvention is a one capable of obtaining stable output characteristic,and industrial utility is great.

I claim:
 1. A temperature detector comprises a light-signal generationpart which generates light-signal, a sensor part located at atemperature measuring part, and optical fiber for signal transmissionfor imputting signal-light to said sensor part, received-light-signalprocessing part which receives light-signal output from said sensorpart, wherein said sensor part comprises:a temperature detection elementconsisting of LiTaO₃ single crystal or Sr_(x) Ba_(1-x) Nb₂ O₆ singlecrystal (0.5<x<1.0), a polarizer of one member selected from a groupconsisting of a rutile and a calcite, for polarizing input light to saidtemperature detection elements, a rod lens disposed between saidpolarizer and said temperature detection element, and a quartz disposedbetween said rod lens and said temperature detection element for settingtemperature range.
 2. A temperature detector of claim 1, wherein saidtemperature detection element has electrodes on surfaces perpendicularto the Z axis of said single crystal, and said electrodes areshort-circuited with respect to each other.
 3. A temperature detector ofclaim 1, wherein said light-signal generation part has plural lightsource of different wavelengths, said received light-signal processingpart has plural processing circuits for respective light-signal outputsof respective wavelengths, and said sensor part has reflection filmwhich reflect only light of particular wavelength on a light emissionpart of said rod lens.